My situation is similar, but a little more perilous I fear: I accidentally started to install XP onto the same partition as Vista (it didn't give me any other partition options despite my having created a 15Gig primary partition for it, just went ahead and chucked it onto C, and I really need to get rid of it without having to reformat the drive and reinstall Vista - is there any way of doing so?
(I didn't get to the point where I fully completed the XP set-up, but when I boot into Vista there's a folder on the C: drive called Windows.0 which I think is the XP, but I'm assuming there are extra bits and pieces - registry etc - that I'd need to delete as well)

Unfortunately there is currently no system restore point otherwise this would be somewhat easier... help!